The present invention relates to a method and/or architecture for crystal oscillators generally and, more particularly, to a method and/or architecture for instantaneous start up crystal oscillators.
Conventional oscillators cannot generate an accurate system clock during a start up condition. In particular, when time keeping accuracy, frequency stability with respect to time and quick start up is required, conventional oscillators are not adequate.
In applications using micro controllers, if an oscillator has not started up when a power-on-reset (POR) is lifted, the microcontroller can hang. A watchdog timer (WDT) has to be implemented to return the system to a normal mode of operation. Using the watchdog timer will cause significant error (i) if the system is used for time measurement from power up and/or (ii) if an application cannot tolerate the inevitable blackout associated with recovery driven by the watchdog timer.
The present invention concerns an apparatus comprising a first oscillator, a second oscillator and a logic circuit. The first oscillator circuit may be configured to generate a first clock signal. The second oscillator circuit may be configured to generate a second clock signal. The logic circuit may be configured to generate an output clock signal by selecting either the first clock signal or the second clock signal.
The objects, features and advantages of the present invention include providing a method and/or architecture for generating an instantaneous frequency during a start up condition that may (i) provide an accurate oscillator frequency after a power start up, (ii) link an RC oscillator based power on reset (POR), (iii) implement a POR that does not lift before the oscillator starts up, (iv) reduce usage of a watchdog timer (WDT) for reset related system recovery, (v) provide for graceful (e.g., non-abrupt) degradation in environments where a crystal oscillator (a) stops, (b) stalls and/or (c) fails (e.g., high xe2x80x9cgxe2x80x9d and high background EMI applications), (vi) provide high accuracy power levels, (vii) provide high accuracy over wide voltage and temperature variations, and/or (viii) provide a robust solution for applications that require microprocessors to receive or respond to external events from power up.